It is known that exposure of tissue to ionizing radiation will kill the cells exposed. In the process of conventional radiation therapy, however, significant volumes of normal tissue in addition to pathological tissue, are exposed to harmful levels of radiation.
Several methods have been employed in the prior art to minimize the exposure of healthy tissue to ionizing radiation. For example, devices which direct radiation at the tumor from a number of directions have been used. In such devices, the amount of ionizing radiation emanating from each source of radiation is less than that which is necessary to destroy tissue. Rather, tissue destruction occurs where the radiation beams from multiple sources converge, causing the radiation level to reach tissue-destructive levels. The point of convergence of the center of multiple radiation beams is referred to herein as the "focus point." The radiation field surrounding a focus point is herein referred to as the "focus volume." The size of the focus volume can be varied by varying the size of the intersecting beams.
One such radiation appliance sold under the name GAMMA KNIFE (Elekta Instruments S.A.) comprises an ionizing radiation shield having a substantial number of ionizing radiation sources. Radiation passes through a number of channels all of which lead toward a common focus point in a recess within the radiation shield. Such a system is referred to, and described in, U.S. Pat. No. 4,780,898. Another system commonly termed a LINAC (or linear accelerator) involves an ionizing radiation source which moves circumferentially around a focus point delivering a series of beams of ionizing radiation through the focus volume. A patient's head, immobilized in a stereotactic instrument which defines the location of the treatment target in the patient's head, is secured by a system which positions the treatment target in coincidence with the above-mentioned focus point.
The ionizing radiation in the focus volume of these radiation appliances is intense compared to the radiation emanating from each individual beam of the device. Areas outside of the focus volume receive less substantial amounts of ionizing radiation. Therefore, pathological tissue can be treated while avoiding surrounding healthy areas.
In general, the focus volume is spherical because the intersection of multiple radiation beam cross sections approximately form a sphere of constant radiation density at each point equidistant from the focus point. As a result, when the shape of the pathological tissue volume is not substantially spherical, either some areas of pathological tissue do not receive enough radiation or other areas of healthy tissue receive too much radiation. In other words, variations in radiation sensitivity within the focus volume cannot be taken into account. To ensure that the whole volume of pathological tissue is fully exposed to the radiation field, the radiation team is obliged to deliver damaging doses of radiation to healthy tissue within the focus volume.
It is possible to reduce the volume of healthy tissue receiving high ionizing radiation doses by reducing the size of the focus volume and manually repositioning the patient a number of times such that the different positions of the various focus volumes would effectively cover the entire pathological tissue volume. While this method allows increased conformity between pathological tissue volume and shape and the volume receiving high radiation doses, the time required to manually reposition a patient a sufficient number of times for the selected focus volume size to effectively cover the pathological tissue can require unreasonably long treatment periods. Moreover, each manual reposition introduces the potential for mistakes with resultant increased radiation of healthy tissue.
A second potential means for minimizing the irradiation of healthy tissue would be to vary individual beam sizes and intensities whereby the shape of the focus volume could be modified to conform more accurately with the pathological tissue volume. With the many possible combinations of incident beam sizes and intensities to be interactively evaluated by the radiology team in order to find a radiation dose distribution appropriate for treating a pathological tissue volume having a specific shape, the experience of the radiotherapy team in choosing the beam sizes and intensities becomes a significant factor in the efficiency and effectiveness of the radiation treatment.
A further solution involves the projection of a focus volume of ionizing radiation onto a treatment area. Such a technique is described in, for example, Experimental Verification of an Algorithm for Inverse Radiation Therapy Planning, Radiotherapy and Oncology, 17 (1990) 359-368. According to this article, it is impractical to move the patient with respect to a fixed focus point. This conclusion was based on Therapy Planning and Dosimetry for the Pion Applicator at the Swiss Institute for Nuclear Research, Radiation and Environmental Biophysics, 16, 205-209 (1979), which was reported to have demonstrated that dynamic movement of the patient in a pion generator was not feasible.
Thus, although the prior art suggests radiation treatment of an object in which the dose distribution closely conforms to the treatment area within the object, the methods are dependent on the skill and experience of the radiology team, involve potential errors during manual repositioning and/or require prolonged treatment times. In contrast to these prior art methods, the present invention--by means of automatic positioning and repositioning of a target area relative to a focus volume--eliminates the risks of manual error, allows use of smaller focus volumes, thereby improving conformity between a radiation field and a target volume and reducing the need for trial and error approach associated with multiple size focus volumes, and shortens the treatment planning time. In addition, contrary to the teaching of the prior art, the present invention permits dynamic movement of an object relative to a radiation source, whereby greater local conformity of dose delivery to pathological tissue volume and shape becomes possible by movement at rates which modulate radiation deposition based on the tissue cellular properties such as radiation sensitivity both inside and outside of the target volume.